1. Field of the Invention
The present invention relates to an organic light emitting diode (OLED) display device, and more particularly, to an OLED display device having a passivation layer over and under a first pixel electrode to prevent corrosion of the first pixel electrode.
2. Description of the Related Art
Generally, in OLED display devices, electrons and holes are respectively injected into an emission layer from a cathode (an electron injection electrode) and an anode (a hole injection electrode) and then combined in the emission layer to create excitons, and when the electrons and holes transition from an excited state to a ground state, light is emitted. By such a principle, the OLED does not require a separate light source, which is required in a conventional thin film liquid crystal display device, so that its volume and weight can be reduced.
The OLED display device may be classified into a passive-matrix type and an active-matrix type depending on its driving mechanism. The passive-matrix OLED display device has a relatively simple configuration, and fabrication method does not require complicated processes, but the passive-matrix OLED display device has disadvantages in power consumption and size. Also, in the passive-matrix OLED display device, an aperture ratio is reduced as the number of interconnections is increased. Therefore, while a small display device employs the passive-matrix OLED, a large display device employs the active-matrix OLED.
Meanwhile, a common top-emission OLED display device employs a reflective electrode with excellent reflectivity at its one side, and the reflective electrode is formed of a conductive material having an appropriate work function in addition to the reflectivity. However, since there is no material having both characteristics so far, the reflective electrode is commonly fabricated in a multi-layered structure in which a reflective layer is separately formed, and an electrode material having a different conductivity is formed on the reflective layer.
Conventional art will now be described with reference to the accompanied drawings.
FIG. 1A is a cross-sectional view of a conventional OLED display device. FIG. 1B is an enlarged cross-sectional view of part A of FIG. 1A, which illustrates that an oxide layer is formed at an interface between a reflective layer and a transparent electrode layer. FIG. 2 illustrates non-uniformity of brightness in the conventional OLED display device.
Referring to FIG. 1A, the OLED display device has a structure in which a reflective layer 20a and a transparent electrode layer 20b, as a pixel electrode 20, are sequentially stacked on a substrate 10, and an organic layer 40 and a counter electrode 50 are sequentially stacked on the resultant structure.
In the OLED display device having such a structure, the reflective layer 20a is uniformly formed of a metallic material with excellent reflectivity on the substrate 10 by sputtering or vacuum evaporation. As the conventional reflective layer, an active metal such as aluminum (Al) or its alloy is used.
The transparent electrode layer 20b is formed by depositing a transparent electrode material on the reflective layer 20a such that light entering into the transparent electrode 20b is reflected by the reflective layer 20a, and then the transparent electrode 20b is formed into a pattern to form a plurality of pixel electrodes. Here, the transparent electrode material includes indium tin oxide (ITO) or indium zinc oxide (IZO).
A pixel defining layer 30 defining a pixel region is formed at both sides of the pixel electrode 20, and the organic layer 40 including an emission layer and the counter electrode 50 are formed on the pixel defining layer 30, and thus a top-emission OLED display device is completed.
In the manufacturing process of the OLED display device described above, the pixel electrode 20 is generally patterned by continuously performing photolithography and etching processes. Specifically, a photoresist pattern is formed on the transparent electrode layer 20b, and exposed and developed, and then the transparent electrode layer 20b and the reflective layer 20a are sequentially etched using the photoresist pattern as a mask.
Here, the etching process may be performed by a wet or dry etching technique, which is generally used. In the wet etching, a strong acid solution such as HF, HNO3 or H2SO4 is applied or sprayed onto a region to be etched to obtain a desired pattern, and the above described strong acids, and other strong acidic and basic chemicals such as HNO3, HCl, H3PO4, H2O2 and NH4OH are used in subsequent cleaning and stripping processes.
The strong acidic and basic chemicals used in the cleaning and stripping processes are in direct contact with the transparent electrode layer 20b and the reflective layer 20a used as the pixel electrodes 20, and thus a metal oxide layer 20c is created at an interface between the transparent electrode layer 20b and the reflective layer 20a as illustrated in FIG. 1B. Particularly, aluminum (Al) and its alloy easily corrode to form the metal oxide layer 20c even when exposed to the air.
Thus, the metal oxide layer 20c increases a sheet resistance of the transparent electrode layer 20b, and is diffused along the interface between the transparent electrode layer 20b and the reflective layer 20a, thereby abruptly increasing a contact resistance between the electrodes and showing very unstable resistance distribution.
Also, as shown in FIG. 2, the brightness of the pixels is not uniform. Colors of some pixels are bright, while those of other pixels are dark. This non-uniform brightness occurs during driving of the OLED display device due to the unstable resistance distribution, and thus the quality of the display significantly degenerates.